1. Field of the Invention
The present invention relates to an endoscope system and a method for controlling an endoscope actuator. Particularly, the present invention relates to an endoscope system having an actuator with a shape memory element for moving an objective optical system, and a method for controlling an endoscope actuator.
2. Description of the Related Art
Endoscope systems have been widely used in medical and industrial fields. An image of an object is picked up with an image pickup device provided in a distal end portion of an endoscope insertion section, and displayed on a monitor apparatus. A person such as a surgeon can perform observation with reference to the object image displayed on the monitor. The image pickup device and an observation optical system are internally provided in the distal end portion of the endoscope insertion section.
In recent years, as disclosed in Japanese Patent Application Laid-Open Publication No. 2009-148369, an endoscope apparatus has been proposed in which a mechanism for a focusing function for an object image is provided in the insertion section to cause a lens frame of the observation optical system to be moved in the optical axis direction. The apparatus uses a shape memory alloy as an actuator for moving the lens frame.
The shape memory alloy wire is elongated and contracted by controlling a current passed through the wire. For example, the shape memory alloy wire produces heat in response to passage of the current and is contracted at high temperatures; whereas it releases heat in response to shutoff of the current and is elongated consequently. Such characteristics of the shape memory alloy are utilized to realize the focusing function of the observation optical system.
In regard to an actuator with a shape memory alloy, Japanese Patent Application Laid-Open Publication No. 2010-48120, for example, discloses an actuator system. The proposed actuator system stores maximum and minimum resistance values observed when a current is passed through a shape memory alloy wire, and performs resistance control based on a resistance value of the shape memory alloy wire.
The above actuator system performs calibration at startup of the actuator system by detecting the minimum resistance value within the moving range of an actuator. A correction value resulting from the calibration is used to perform the resistance control.